Reverse timestamp method and network node for clock recovery

1. A method of clock recovery of a Time Division Multiplex signal in a packet network comprising: a) receiving by a first Provider Edge a timing message from a second Provider Edge, wherein a counter in said first Provider Edge is increased upon reception of the timing message; b) sending by the first Provider Edge to the second Provider Edge encapsulated Time Division Multiplex packet traffic, wherein the counter is decreased upon sending a packet to the second Provider Edge; c) comparing the counter with an Upper Threshold value; d) increasing a rate of generation of the encapsulation packets if the counter is above said Upper Threshold; e) comparing the counter with a Low Threshold value; and f) reducing the rate of generation of encapsulation packets if the counter is below said Low Threshold.

2. The method according to claim 1 wherein the rate of generation of encapsulation packets is unchanged if the counter is between said Low Threshold and said Upper Threshold.

3. The method according to claim 1 , wherein the rate of generation of encapsulation packets depends on the encapsulated payload size output from a FIFO buffer of the first Provider Edge and if the counter increases above the Upper Threshold the first Provider Edge reduces the payload size and if the counter decreases below the Low Threshold the first Provider Edge increases the payload size.

4. The method according to claim 1 , comprising adding to the encapsulation packet justification information indicating how the payload size of the packet differs from a default payload size.

5. The method according to claim 1 comprising reading by the first Provider Edge a timestamp from the received timing message, comparing it with a timestamp from the previously received timing message, calculating time between these two timestamps and correcting the counter if the time period between these two timestamps is bigger than the time period between two consecutive timestamps.

6. The method according to claim 5 , wherein the timing message comprises information about a difference between a writing address and a reading address of an input FIFO buffer of the second Provider Edge.

7. The method according to claim 6 , wherein if it is detected at the first Provider Edge that the difference between the writing address and the reading address of the input FIFO buffer of the second Provider Edge is smaller than a first predefined value the first Provider Edge reduces the payload size and increases the payload size if said difference between the writing address and the reading address is bigger than a second predefined value.

8. The method according to claim 1 , wherein the size of the encapsulation packet is constant.

9. The method according to claim 4 , wherein the encapsulation packet contains information about justification applied in all previous encapsulation packets sent from the first Provider Edge to the second Provider Edge in the current communication session.

10. A communications network node operable in Time Division Multiplex, the communications network node comprising: a message elaboration unit for receiving a timing message from a second Provider Edge; a message encapsulation unit operable to encapsulate Time Division Multiplex traffic into packets and to send these encapsulation packets to the second Provider Edge; a counter operable to be increased upon reception of the timing message and decreased upon sending the encapsulation packet; and a first comparison unit operable to compare the counter with an Upper Threshold value and with a Low Threshold value, wherein the node is operable to increase a rate of generation of the encapsulation packets if the counter is above said Upper Threshold or to reduce the rate of generation of encapsulation packets if the counter is below said Low Threshold.

11. The node according to claim 10 wherein the node is operable to keep the rate of generation of encapsulation packets unchanged if the counter is between said Low Threshold and said Upper Threshold.

12. The node according to claim 10 comprising a FIFO buffer, wherein the rate of generation of encapsulation packets depends on the encapsulated payload size output from said FIFO buffer and the node is operable to reduce the payload size if the counter increases above the Upper Threshold and to increase the payload size if the counter decreases below the Low Threshold.

13. The node according to claim 10 , wherein the encapsulation unit is operable to add to the encapsulation packet a justification information indicating how the payload size of the packet differs from a default payload size.

14. The node according to claim 10 , wherein the message elaboration unit is operable to read a timestamp from the received timing message and to compare it with a timestamp from the previously received timing message, the message elaboration unit is further operable to calculate a time period between these two timestamps and correct the counter if the time period between these two timestamps is bigger than the time period between two consecutive timestamps.

15. The node according to claim 14 , wherein the timing message comprises information about a difference between a writing address and a reading address of an input FIFO buffer of the second Provider Edge.

16. The node according to claim 15 , wherein the encapsulation unit is operable to reduce the payload size if the message elaboration unit detects that the difference between the writing address and the reading address of the input FIFO buffer of the second Provider Edge is smaller than a first predefined value and operable to increase the payload size if said difference between the writing address and the reading address is bigger than a second predefined value.

17. The node according to claim 10 , wherein the size of the encapsulation packet is constant.

18. The node according to claim 13 , wherein the encapsulation packet contains information about justification applied in all previous encapsulation packets sent from said node to the second Provider Edge in the current communication session.

19. A communications network operable in Time Division Multiplex, the communications network comprising: at least a first Customer Edge connected to a first Provider Edge; and a second Customer Edge connected to a second Provider Edge, wherein: the connections between the Customer Edges and the Provider Edges operate in Time Division Multiplex and the first Provider Edge is operable to receive a timing message from the second Provider Edge; a counter in said first Provider Edge is increased upon reception of the timing message; the first Provider Edge is also operable to send to the second Provider Edge encapsulated Time Division Multiplex packet traffic, wherein the counter is decreased upon sending an encapsulation packet to the second Provider Edge and the first Provider Edge is operable to compare the counter with an Upper Threshold value and with a Low Threshold value, and to increase a rate of generation of the encapsulation packets if the counter is above said Upper Threshold or to reduce the rate of generation of encapsulation packets if the counter is below said Low Threshold.

20. A communications network node operable in Time Division Multiplex, the communications network node comprising: a timing message generation unit for sending timing messages; a clock; a counter running with said clock and the value of the counter being added to the timing messages in said timing message generation unit; a receiving FIFO buffer for receiving encapsulation packets from a communications network node as defined in claim 11 ; an address generator operable to generate read addresses of the receiving FIFO buffer; a gap circuit connected to the receiving FIFO buffer and to a second FIFO buffer, wherein the gap circuit is operable to forward clock edges in positions of payload and masking other clock edges; and a Phase Locked Loop unit operable to recover from the gapped clock signal a clock signal used for timing the payload encapsulated in said encapsulation packets.